Experiment 29: Filtering Frequencies
252 Chapter 5
The 33K resistor is necessary to protect the amplifier from being overdriven. If
you don’t get enough volume using your music player, decrease the 33K value.
If the music is too loud and distorted, increase the value. You can also try omit-
ting or increasing the 10K resistor next to it, which is included in an effort to
reduce background hum noise.
I’ve shown two switches at the top of the schematic: one to bypass a coil, the
other to bypass a capacitor. You can use alligator clips instead, as long as you
can easily compare the sound when each of the components is inserted into
the circuit.
Figure 5-43 shows a coil consisting of a spool of hookup wire being used. The
red and black alligator clips resting loose on top of the shoebox will go to the
output from the chip (on pins 2 and 15). There is no polarity; it doesn’t matter
which clip goes to which pin.
Begin by turning the volume control on your music source all the way down
before you apply power. Don’t be surprised if you hear humming or crackling
noises when you activate the amplifier; it will pick up any stray voltages, be-
cause in this simple experiment, I haven’t suggested that you should shield
the input, and the amplifier circuit can pick up noise, as its wires can act like
antennas.
Note that you may also get additional unwanted sound if you use the ampli-
fier on a conductive desktop surface. Remove any aluminum foil or conductive
foam for this project.
Make sure that your player is playing music, and slowly turn up its volume
control until you hear it. If you don’t hear anything, you’ll have to check for
circuit errors.
Now comes the interesting part. Insert the 100-foot spool of hookup wire be-
tween one output from the amplifier, and one input of the speaker (it doesn’t
matter which one), or if you used switches, open the switch that bypasses the
coil. You should find that the music loses all its high-end response. By compari-
son, if you disconnect the coil and substitute a 10 μF capacitor, you should find
that the music sounds “tinny,” meaning that it loses all its low range, leaving
only the high frequencies.
You’ve just tested two very simple filters. Here’s what they are doing:- The coil is a low-pass filter. It passes low frequencies but blocks high fre-
quencies, because brief audio cycles don’t have time to overcome the coil’s
self-inductance. A bigger coil eliminates a wider range of frequencies. - The capacitor is a high-pass filter. It passes high frequencies and blocks
low frequencies because longer audio cycles can fill the capacitance, at
which point the capacitor stops passing current. A smaller capacitor elimi-
nates a wider ranger of frequencies.
You can go a lot farther into filter design, using complex combinations of coils
and capacitors to block frequencies at any point in the audible spectrum.
Search online for audio filter schematics—you’ll find hundreds of them.
Figure 5-43. The red and black alligator
clips, lying on top of the shoebox, should
connect with the output from your ampli-
fier chip. The red jumper wire passes the
signal through a coil of hookup wire on its
way to the speaker. Note the change in
sound when you short out the coil.